1. Field of the Invention
The present invention relates to a tensioning mechanism, and more particularly to a saw chain tensioning mechanism for changing a longitudinal position of a chain guide plate of a chain saw relative to a main body case of the chain saw to adjust a tension of a saw chain wound around the chain guide plate
2. Description of the Related Art
As one type of saw chain tensioning mechanism, there has heretofore been known a saw chain tensioning mechanism as disclosed, for example, in Japanese Patent Laid-Open Publication No. 10-286802. FIG. 6 of the present application illustrates such prior art and shows a saw chain tensioning mechanism 50 incorporated in a chain saw 58 to adjust a longitudinal position of a chain guide plate 56 which has a rear end portion supported between a side case 52 and a main body case 54 through the use of a stud bolt 67 and a clamp nut 68, and protrudes frontward from the cases. The saw chain tensioning mechanism comprises a threaded rod member 62 having a first end formed with a tool insertion groove 60, and a pin member 64 adapted to be moved along the axial direction of the threaded rod member by a rotation of the threaded rod member so as to change the longitudinal position of the chain guide plate. The threaded rod member is provided with a driven bevel gear fixed to a second end thereof on the opposite side of the first end, and the driven bevel gear is engaged with a driving bevel gear having a shaft rotatably supported by the side case. Further, the shaft of the driving bevel gear is formed with a tool insertion groove 60a exposed outside the side case to allow an operator to perform an operation for rotating the threaded rod member using a tool from the side of the chain saw.
In the above conventional saw chain tensioning mechanism, the shaft or the threaded rod member has to be rotated using a tool. However, it is bothersome for an operator to always take along the tool. Moreover, if an operator forgets about taking along the tool, the adjustment cannot be performed in a working field. Thus, there is a need in the art for ways and mechanisms for adjusting tension in a saw chain without a tool.